The present invention relates generally to aluminum alloy sheet and methods for making aluminum alloy sheet and specifically to aluminum alloy sheet and methods for making aluminum alloy sheet for use in forming drawn and ironed container bodies.
Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (referred to herein as a xe2x80x9ccontainer bodyxe2x80x9d) and a second piece forming a container top. Container bodies are formed by methods well known in the art. Generally, the container body is fabricated by forming a cup from a circular blank aluminum sheet (i.e., body stock) and then extending and thinning the sidewalls by passing the cup through a series of dies having progressively smaller bore sizes. This process is referred to as xe2x80x9cdrawing and ironingxe2x80x9d the container body. The ends of the container are formed from end stock and attached to the container body. The tab on the upper container end that is used to provide an opening to dispense the contents of the container is formed from tab stock.
Aluminum alloy sheet is most commonly produced by an ingot casting process. In the process, the aluminum alloy material is initially cast into an ingot, for example, having a thickness ranging from about 20 to about 30 inches. The ingot is then homogenized by heating to an elevated temperature, which is typically 1075xc2x0 F. to 1150xc2x0 F., for an extended period of time, such as from about 6 to about 24 hours. xe2x80x9cHomogenizationxe2x80x9d refers to a process whereby ingots are raised to temperatures near the solidus temperature and held at that temperature for varying lengths of time. The process reduces microsegregation by promoting diffusion of solute atoms within the grains of alumina and improves workability. Homogenization does not alter the crystal structure of the ingot. The homogenized ingot is then hot rolled in a series of passes to reduce the thickness of the ingot. The hot rolled sheet is then cold rolled to the desired final gauge.
Although ingot casting is a common technique for producing aluminum alloy sheet, a highly advantageous method for producing aluminum alloy sheet is by continuously casting molten metal. In a continuous casting process, molten metal is continuously cast directly into a relatively long, thin slab and the cast slab is then hot rolled and cold rolled to produce a finished product.
Some alloys are not readily cast using a continuous casting process into an aluminum sheet having mechanical properties suitable for forming operations, especially for making drawn and ironed container bodies. By way of example, some alloys have low yield and tensile strengths, a low degree of formability and/or a high earing which lead to a number of problems.
It would be desirable to have a continuous aluminum casting process in which the aluminum alloy sheet can be readily fabricated into desired objects. It would be advantageous to have a continuous casting process in which the aluminum alloy sheet has a high degree of formability, low earing and high strength.
These and other needs are addressed by the process and alloy compositions of the present invention. In a first embodiment, the method can include the steps of:
(a) continuously casting an aluminum alloy melt to form a cast strip;
(b) hot rolling the cast strip to form a hot rolled strip;
(c) cold rolling the hot rolled strip to form an intermediate cold rolled strip;
(d) continuously annealing the intermediate cold rolled strip at a temperature ranging from about 371 to about 565xc2x0 C. to form an intermediate annealed strip; and
(e) cold rolling the intermediate cold rolled strip to form aluminum alloy sheet.
The use of a continuous anneal can provide significant savings in operating and alloy costs and improvements in production capacity. As will be appreciated, batch anneals require a significantly increased amount of labor to perform, and batch anneal ovens have a limited capacity.
The continuous annealing step (d) is preferably conducted in an induction heater with a transflux induction furnace being most preferred. The annealing step (d) surprisingly yields an intermediate annealed strip having mechanical properties (i.e., yield tensile strength and ultimate tensile strength) that can be selectively controlled by varying the temperature and duration of a later stabilizing or back annealing step (collectively referred to as a xe2x80x9cstabilizing annealxe2x80x9d). For the induction furnace, the residence time of any portion of the cold rolled strip in the continuously annealing step (d) ranges from about 2 to about 30 seconds.
It has been discovered that induction heaters can provide aluminum alloy sheet having not only a finer grain size but also a substantially uniform distribution of the finer grain size throughout the coil formed by the intermediate annealed strip. The relatively fine grain size can provide not only more uniform mechanical properties throughout the coil but also mechanical properties that are controllable by varying the temperature and duration of a later stabilizing or back annealing step.
The induction furnace can be superior to radiant furnaces in annealing aluminum alloys because the induction furnace more uniformly heats the strip. Radiant furnaces place the strip in a heated atmosphere and rely on thermal transfer to anneal the entire cross-section of the strip, which can lead to more exposure of the exterior portions of the strip/coil to heat and less exposure of the middle of the strip/coil to heat. In contrast, induction furnaces use electromagnetic energy to heat the strip substantially uniformly throughout the strip""s cross-section. Accordingly, induction heaters can provide for greater gains in mechanical properties through annealing than radiant heaters and, therefore, permit the use of lower amounts of expensive alloying elements to realize selected mechanical properties.
Aluminum alloy sheet produced by this process is especially useful as body stock in canmaking applications. To provide the desired low earing for container manufacture, cold rolling step (c) can be used to produce a relatively large reduction in the gauge of the strip while cold rolling step (e) is used to produce a relatively low reduction in the gauge of the intermediate cold rolled strip (i.e., a low amount of work hardening). The low amount of work hardening can produce a concomitant relatively low increase in yield and ultimate tensile strengths. The yield and ultimate tensile strengths can then be increased to desired levels in a later stabilizing annealing step by selecting the appropriate annealing or back temperature and time, without a significant increase in earing.
Other embodiments of the method employ the induction furnace in annealing steps performed after hot rolling, such as in a stabilizing anneal. The unique performance advantages of the induction furnace can provide highly desirable mechanical properties in the aluminum alloy sheet which can be controlled in later annealing steps as noted above.
In a particularly preferred process for producing aluminum sheet useful as body stock, a number of additional steps. The complete process includes the following steps:
(a) continuously casting an aluminum alloy melt to form a cast strip having a cast output temperature;
(b) heating the cast strip, either before hot rolling or after partial hot rolling, to a heated temperature that is from about 6 to about 52xc2x0 C. more than the cast output temperature to cause later recrystallization of the cast strip after step (c) below;
(c) hot rolling the cast strip to form a hot rolled strip;
(c) cold rolling the hot rolled strip to form an intermediate cold rolled strip;
(d) intermediate annealing of the intermediate cold rolled strip in an induction furnace at a temperature ranging from about 371 to about 565xc2x0 C. to form an intermediate annealed strip; and
(e) cold rolling the intermediate cold rolled strip to form aluminum alloy sheet.
After step (e), the aluminum alloy sheet can be subjected to a stabilizing anneal, as desired, to provide desired mechanical properties. xe2x80x9cRecrystallizationxe2x80x9d refers to a change in grain structure without a phase change as a result of heating of the strip above the strip""s recrystallization temperature.
An alloy useful in this process for producing body stock has the following composition:
(i) from about 0.9 to about 1.5% by weight magnesium,
(ii) from about 0.8 to about 1.2% by weight manganese,
(iii) from about 0.05 to about 0.5% by weight copper,
(iv) from about 0.05 to about 0.5% by weight iron, and
(v) from about 0.05 to about 0.5% by weight silicon.
Body stock produced using this alloy and process can have particularly attractive properties. By way of example, the aluminum alloy sheet can have an as-rolled yield strength of at least about 38 ksi, an as-rolled tensile strength of at least about 42.5 ksi, an earing of less than about 1.8%, and/or an elongation of at least about 3%. As will be appreciated, xe2x80x9cearingxe2x80x9d is typically measured by the 45 degree earing or 45 degree rolling texture. Forty-five degrees refers to the position of the aluminum alloy sheet which is 45 degrees relative to the rolling direction. The value for the 45 degree earing is determined by measuring the height of the ears which stick up in a cup, minus the height of valleys between the ears. The difference is divided by the height of the valleys and multiplied by 100 to convert to a percentage. Surprisingly, strip that is intermediate annealed using an induction heater generally has as-rolled yield and tensile strengths that are about 3 to about 5 ksi more than that of a strip that is intermediate annealed using a batch heater.
Container bodies produced from the body stock can also have superior properties. Container bodies produced from aluminum alloy sheet can have a buckle strength of at least about 90 psi and a column strength of at least about 180 psi.